John Andersen, 53, is the man behind the microphone in the Boston Marathon finish line medical tent. His job has nothing to do with bandages or blood. He directs volunteers to where they’re needed, keeping people moving and keeping the mood light.

Never was that role more necessary than last week, when a tent designed to treat hypothermia and dehydration transformed into nothing short of a battlefield hospital.

And while patients passed him with mangled limbs, no limbs, or bodies peppered with shrapnel from the two homemade bombs that blew up along Boylston Street, Andersen kept calm and kept doing what he has done for the past 15 years.

"Please, take care of your patients," he told the volunteers.

Other than that, Andersen can’t tell you what he said that day. He just remembers how the doctors, nurses and other volunteers fell into place and worked seamlessly.

"It was almost an eerie silence inside the tent, as those professionals who were not being micromanaged, who were not thinking of themselves, selflessly did whatever they could," he said.

The only noise was the sound of ambulances descending from all parts of the city into Copley Square.

Every patient who came into the tent alive, left alive. That’s nothing short of miraculous, Andersen says.

Outside the door stood his wife Laurie, triaging patients. She usually sees blisters, hypothermia and dehydration. A week ago she saw every bombing victim who entered the tent, an estimated 50 to 60 people.

As head nurse of the Newton-Wellesley Hospital emergency room, not a lot fazes her. But Andersen, a middle school science teacher, still can’t shake the memory of the deep brown color of blood shellacked on people.

He can still see the grey, ghostly face of Jeff Bauman, the man who lost both his legs, and who was captured in a photo as he was rushed, in a wheelchair, toward the medical tent.

But through the crush of victims pouring into the tent, Anderson's voice never wavered. He did what he has done for 15 years, keeping people moving and the mood as light as possible.

"Despite the sights that he saw, he stood right there and stayed there to keep us calm," said Jeanette Corsini, a Hopkinton nurse who runs the medical tent.

Andersen and his wife, along with many of the medical tent staff, are meeting with counselors hired by the Boston Athletic Association.

"I think we’re still in shock but doing better a week later," he said.

They know what they saw will be seared in their minds forever. They won't forget what they did. But they are more eager than ever to be back at their posts next year.